Combustion air and fuel are combined at a burner, mixed and ignited there and burned in a flame. It is generally very important to minimize the emission of pollutants, such as carbon monoxide and nitrogen oxide. Combustion with low levels of nitrogen oxide can be achieved in particular with so-called premix combustion, in which fuel and combustion air are first mixed as homogenously as possible, before being fed to the combustion zone. Such a premix burner is disclosed in WO 02/095293 A1. This burner has an annular premixing channel around a central diffusion burner. Helical blades are disposed in the premixing channel in a helical grid running across the entire cross-section of the premixing channel significantly upstream from the combustion zone. Such a helical grid serves to stabilize the flame. The helical blades of the disclosed helical grid are configured in a hollow manner, with holes extending in a radial direction with helical blades on the surface of the helical blades. Fuel is admitted into the premixing channel from these holes, having been previously fed to the hollow helical blades. This achieves a uniform admission of fuel into the combustion air flowing through the premixing channel over the radial height of the premixing channel. At the same time the admission of fuel from all the helical blades achieves a uniform distribution of fuel in the peripheral direction of the premixing channel. This results in a high level of homogeneity of the combustion air/fuel mixture then flowing into the combustion zone. Such homogeneity is desirable for low nitrogen oxide emissions, as the formation of nitrogen oxide increases exponentially with the flame temperature. A homogenous mixture prevents local peak temperatures, as energy is released in a uniform manner in the mixture. This reduces the formation of nitrogen oxide. Also comparatively little fuel is burned in the combustion air during premix combustion. This so-called lean premix combustion however tends to produce combustion instabilities, i.e. the energy released by the flame fluctuates or the flame can even be extinguished. The central diffusion burner, at which fuel and combustion air are mixed in the flame, serves to stabilize this premix combustion. To achieve further stabilization of the flame stability of the premix flame, it is proposed that flow-blocking elements be provided from the outer edge of the annular premixing channel, to delay the flow of combustion air at certain local points. This results in these zones in an enrichment of the combustion air/fuel mixture with fuel, thus giving rise to local hot strands in the combustion zone, which stabilize premix combustion. This static concept cannot however take into account conditions that change as a function of operation.